The present invention relates to an optical recording medium, as well as a process for producing the same, and in particular an optical recording medium having a recording layer including an organic dye as the main component, as well as a process for producing the same.
In recent years, various optical recording disks such as recordable type disks or rewritable type disks attract attention as large-capacity information media. Optical recording disks having a recording layer including a dye as the main component can be produced at a low cost because the recording layer can be formed by coating.
The conventionally used optical recording disks having a recording layer including a dye as the main component are the disks of the so-called air-sandwiched structure provided with an air layer on the recording layer, and recently, optical recording disks capable of reproduce by providing a reflective layer so as to contact directly with the surface of a recording layer have been developed for meeting compact disk (CD) standards. That is, these optical recording disks are the disks having a recording layer including a dye as the main component, a reflective layer and a protective layer laminated in this order on a transparent substrate such as polycarbonate, said reflective layer being provided so as to contact directly with the recording layer, to achieve the disk overall thickness of 1.2 mm meeting CD standards.
Further, similar but high-density optical recording disks meeting DVD standards are also proposed.
In optical recording disks with such layer constitution, however, the moisture permeability of polycarbonate used as the substrate is high, so there is a problem that in a high humidity atmosphere, water permeates into the dye layer and deterioration of the dye proceeds in the presence of water.
The techniques of preventing water from permeating into a dye layer (or amorphous alloy thin layer) as the recording layer have been proposed heretofore. For example, Japanese Laid-Open Patent Publication No. 58-62841 describes the technique of providing a hard coat layer on the surface of a substrate, and Japanese Laid-Open Patent Publication No. 59-198545 describes the technique of providing an interlayer of organotitanium between a substrate and an alloy recording layer. Further, Japanese Laid-Open Patent Publication No. 4-102241 describes the technique of providing an interlayer comprising a heterocyclic compound between a substrate and a dye layer, and Japanese Laid-Open Patent Publication No. 4-131280 describes the technique in which a heterocyclic compound is included in a dye layer and a reflective layer comprises 2 layers. However, any techniques described in these publications require an additional layer for protecting the dye layer (or amorphous alloy thin layer), thus increasing the number of steps for production of optical recording disks and resulting in higher manufacturing costs.
Further, Japanese Laid-Open Patent Publication No. 6-231488 describes that the surface of a reflective layer at the side of a protective layer is treated with a triazine thiol type compound to prevent the progress of corrosion of the reflective layer. However, the technique described in this publication also requires an increase in the number of steps for production of optical recording disks and resulting in higher manufacturing costs.
In addition, an ultraviolet-curable resin for the protective layer is examined for preventing the corrosion of the metal layer itself. For example, Japanese Laid-Open Patent Publication No. 1-199339 defines an acid value in ultraviolet-curable resin; Japanese Laid-Open Patent Publication No. 5-36133 defines the amount of an organic acid ingredient in resin after cured; and Japanese Laid-Open Patent Publication No. 7-192315 defines the concentration of acrylic acid ions in the ultraviolet-cured resin layer. However, the object of any techniques described in these publications is to prevent the corrosion of the metal layer itself, and the corrosion of the dye layer is not referred to therein.
In particular, Japanese Laid-Open Patent Publication No. 7-192315 discloses that an ultraviolet-cured resin layer with 800 ppm or less acrylic acid ions is laminated on a thin metal layer formed on a substrate, and said ultraviolet-curable resin is cured, wherein low reflective Al or an Al alloy is used as said thin metal layer, and that the concentration of acrylic acid ions is made 800 ppm or less, whereby partial oxidative deterioration of the reflective layer is prevented. However, when Al or an Al alloy is used as the reflective layer, an optical recording disk with desired performance cannot be obtained because of the low reflectivity of the metal.
Accordingly, when a highly reflective metal or alloy is used as the reflective layer and an organic dye is used in the recording layer, there are still not known any techniques by which this deterioration of organic dye can be easily prevented.
Accordingly, the object of the present invention is to provide an optical recording medium comprising a recording layer including an organic dye as the main component and a reflective layer comprising a highly reflective metal or alloy, as well as a process for producing said optical recording medium, said optical recording medium preventing deterioration of the organic dye, having improved durability and being highly reliable thereby solving the problems in the prior art as described above.
As a result of their eager study, the present inventors found that in an optical recording medium having a recording layer including an organic dye as the main component and a reflective layer comprising a highly reflective metal or alloy, an organic acid contained in an ultraviolet-cured layer laminated on the reflective layer induces the initial stage of deterioration of the dye layer.
Although the ultraviolet-cured resin layer is not contacted directly with the dye layer, it is believed that the organic acid in the ultraviolet-cured resin layer through the reflective layer forms a local cell in the interface between the dye layer and the reflective layer, to induce deterioration of the dye layer.
The present inventors focused their attention on a irradiation dose of UV rays to an ultraviolet-curable material to be laminated and cured on a reflective layer and the organic acid content in the cured resin layer, and they arrived at the present invention.
That is, the present invention relates to an optical recording medium comprising a recording layer and a reflective layer in this order on a substrate, said reflective layer having a protective layer thereon, wherein the recording layer includes an organic dye as the main component, the reflective layer comprises a metal selected from Cu, Ag and Au or an alloy containing at least one of these metals, and the protective layer is formed from an ultraviolet-cured resin which is cured by irradiating an ultraviolet-curable material with UV rays at 200 to 1000 mJ/cm2 in such a manner that a cure shrinkage factor is not more than 10% by volume, and the organic acid content in said cured resin is not more than 10 ppm by weight.
Here, xe2x80x9ccure shrinkage factorxe2x80x9d refers to a ratio by volume of an ultraviolet-cured material shrunk by irradiation with UV rays and is expressed in the equation:
Cure shrinkage factor (%)=[(v1xe2x88x92v2)/v1]xc3x97100
wherein:
v1 is the volume of the ultraviolet-curable material before irradiation with UV rays, and v2 is the volume of the ultraviolet-cured material after irradiation with UV rays.
The organic acid content in the ultraviolet-cured resin is a value quantitated by extracting an organic acid ingredient in pure water and analyzing this organic acid ingredient by ion chromatography.
The organic dye in the recording layer in the optical recording medium of the present invention is preferably a cyanine type dye. Further, the metal or alloy in the reflective layer is preferably a metal selected from Cu and Ag or an alloy containing at least one of these metals. Further, the organic acid content in the ultraviolet-curable material used in forming the protective layer is preferably not more than 200 ppm by weight.
Further, the present invention relates to a process for producing said optical recording medium which comprises forming a recording layer and a reflective layer in this order on a substrate and forming a protective layer on the reflective layer, wherein the recording layer is formed using an organic dye, the reflective layer is formed using a metal selected from Cu, Ag and Au or an alloy containing at least one of these metals, and the protective layer is formed by laminating an ultraviolet-curable material on the reflective layer and then curing the ultraviolet-curable material by irradiation with UV rays at 200 to 1000 mJ/cm2 in such a manner that a cure shrinkage factor is not more than 10% by volume so that the organic acid content in the resulting ultraviolet-cured resin is not more than 10 ppm by weight
In the production process of the present invention, the organic dye used in the recording layer is preferably a cyanine type dye. The metal or alloy in the reflective layer is preferably a metal selected from Cu and Ag or an alloy containing at least one of these metals. Further, the protective layer is formed preferably using an ultraviolet-curable material with an organic acid content of not more than 200 ppm by weight.